Apparatus for the production of an injection molded part

ABSTRACT

A injection molded part of thermoplastic material, in particular an information carrier disk, is produced by cutting and removal of a sprue part from the injection molded part. The sprue removal leaves an opening in the injection molded part. In order to obtain a clean and smooth opening surface with a fit diameter that is as accurate as possible, the sprue part is axially offset relative to the injection molded part so that the integral connection between these two parts extends only over a part of the opening. The diameter of the axially projecting part is equal to or larger than the diameter of the opening and the rest of the opening being produced by injection molding. The dimensions are advantageously chosen so that a parting edge inclined at an angle of 45° to the plane of the opening is formed and that the injection molded part amounts to 70% and the part to be separated to 30% of the length of the open. The injection molded length of the opening may also be variable. The sprue part may be cut from the injection molded part and removed, either in the closed or open state of the injection molding tool.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process and an apparatus for the productionof an injection molded part, in particular an apertured informationcarrier disk.

2. Description of the Related Technology

The sprue funnel serves, by virtue of its specific configuration, tointroduce liquified plastic material arriving from the channel of theinjection molding nozzle into the molded cavity. The sprue funneleffects a uniform distribution around the circumference and diverts theplastic material by 90°. The sprue part formed in the course of theinjection molding process in the sprue cavity must be cut and removedfrom the molding after the cooling phase.

DE-OS No. 28 38 634 shows a device to cut the sprue part, which isintegrally joined to the molding, by punching and ejecting the sprue bya sprue ejector in the production of an annular molding by an injectionmolding process. The two-part injection mold is opened by displacingboth parts of the mold axially from each other to punch out the spruepart. The separation of the two mold parts is effected in two steps,i.e., initially only far enough so that the sprue part which is held inan undercut is punched into the free space created in this manner by ahydraulically actuated tubular ejector, wherein the sprue is still heldby the undercut. Subsequently, the mold is opened completely and thesprue part pushed out of the undercut by an ejector pin guided in thetubular ejector, so that it drops downward in the parting plane of theinjection molding tool.

Precision and freedom of dust requirements are extremely high ininformation carrier disks, such as sound and image carrier disksproduced by an injection molding process. Particular problems arise inthe creation of a center opening by the punching of the sprue part fromthe disk, as shown in DE-OS No. 28 38 634.

The tolerance limits of the fit diameter of the opening are larger thanthose of the other dimensions of the disk due to the punching process.The surface of the punched opening is also rougher than those of therest of the injection molded surface of the disk. The dust particlesnecessarily produced in the process may affect the molded article. Inthe course of the production of information carrier disks dust may bedeposited on the die plate used in the process, which would be embeddedin the product in the next molding step.

A disk produced with embedded dust would exhibit impaired reproductionquality, possibly rendering the disk unusable. The punching processfurther generates stresses which may have a detrimental effect on theproduct.

SUMMARY OF THE INVENTION

It is an object of the invention to shape the opening produced by aninjection molding tool in an injection molded part, in particular aninformation carrier disk, made with a sprue, from which the sprue partis subsequently removed thereby creating said opening. The opening isproduced to exhibit a fit diameter with an optimum of precision withnarrow tolerance limits. The opening surface is of high quality and thestresses prevailing in the area of the opening are as low as possible.

This object is attained in a process for the production of an injectionmolded part of a thermoplastic material, in particular an informationcarrier disk, by means of an essentially two-part injection moldingtool, with a mold cavity to receive and a sprue funnel for the pouringof an injection molded part. The injection molded part is integrallyjoined to a sprue part. After the injection molding and cooling process,an appropriate opening is produced in said injection molded part bysubsequent separation of the sprue part from the injection molded part.In the process the sprue part is axially offset relative to theinjection molded part to the extent that an integral connection betweenthe two parts extends only over a part of the axial length of theopening. The diameter of the axially projecting part of the sprue partis conveniently equal to or larger than the diameter of the opening andthe remaining part of the opening is made by injection molding. In theprocess a major portion of the opening length or surface is produced bythe injection molding method and defined by the mold parts. An accuratefit diameter with a high surface quality is thereby obtained. Theconnecting area between the injection molded part and the sprue part maybe chosen to be small enough so that the cross section of the passage isjust adequate for flow of the liquified plastic material and the partingedge will not interfere with the fit diameter.

In an advantageous embodiment of the invention the axial offset and thediameter of the projecting part of the sprue part are chosen so thatupon cutting or separating the sprue part from the injection molded parta parting edge inclined at an angle of 45° to the plane of the openingis formed and/or the injection molded portion amounts to 70%, and thepart to be removed to 30%, of the axial length of the opening of moldedpart.

An advantageous apparatus for the molding process includes a stationaryhalf and an axially displaceable half with a bushing arranged inside thestationary half. The bushing comprises an internal sprue funnel and hasan external diameter which corresponds to the contour of the opening.The front edge of the bushing extends axially over or past the frontaledge of the stationary half. A sleeve with a conical narrowing formingan undercut in the flange of the sprue part and a further narrowing forthe centered holding of the sprue part is provided inside thedisplaceable half. The internal diameter of the displaceable halfcorresponds to the diameter of an axially projecting part of the spruepart. An axially displaceable ejector for the ejection of the sprue partis guided in the sleeve. According to this embodiment the sprue part maybe cut and removed from the injection molded part either in the open orclosed state of the injection molding tool. The sprue part is separatedfrom the injection molded part by a sleeve with the conical narrowingmoved away from the injection molded part while entraining the ejectorand the sprue part, which is resting against the injection molding tool,is moved along by the undercut formed in the flange of the sprue part.The sprue may be removed with the tool open or closed. In the lattercase, the injection molded part is held securely by its integral jointwith the sprue part, until the latter is removed, so that no specialholding means, such as an undercut on the injection molded part, arerequired.

The sprue part itself is also held securely until the completion of theejection process by the continuing narrowing toward the center spruereceptacle.

The result of the elimination of the punching process is that nopunching edge wear and dust formation is largely avoided. Furthermore,the absence of a punching die leaves more space for cooling in the areaof the injection nozzle and the sprue part.

The larger size of the diameter of the moving half compared with thestationary half of the injection molding tool where the internaldiameter of the displaceable half is larger than the external diameterof the bushing results in an advantageous inclination of the partingedge, where the axial offset and the diameter of the projecting part ofthe sprue part are chosen so that during the separation of the spruepart from the injection molded part a parting edge inclined at an angleof 45° to the plane of the opening is formed.

In an advantageous embodiment the bushing arranged within the stationaryhalf of the injection molding tool has an outer diameter whichcorresponds to the contour of the opening and is axially displaceableand retainable. The injection molded opening length may be varied byaxial displacement of the bushing, as for example, where the injectionmolded part amounts to approximately 70% and the part to be separated toapproximately 30% of the axial length of the part opening.

The arrangement of the bushing in an axially displaceable fashionenables its use as a pressure piston in order to press the sprue partfrom the injection molded part.

The drawings show an exemplary embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a longitudinal section through an injection molding toolwith an injection molded part and a sprue part.

FIG. 2 shows a segment A according to FIG. 1, enlarged.

FIG. 3 shows a sectional view according to claim 1, illustrating thecutting of the sprue part from the injection molded part.

FIG. 4 shows the axially displaceable left hand part of the injectionmolding tool with the sprue part cut, immediately prior to ejection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The injection molding tool shown as a segment in FIG. 1 includes astationary half 1 and an axially displaceable half 2 with an internaldiameter or surface 18. A cavity is formed between the two halves filledwith a sprue part 3 with a flange 12 and an injection molded or productpart 4 with an opening 5. The sprue part and the product part areintegrally joined together as a result of the injection molding processat a connecting location 6.

An axially displaceable and retainable bushing 7 with an externaldiameter or surface 20 is arranged within the stationary half 1. Itincludes an integral sprue funnel 8. Its outer diameter 20 correspondsto the contour of the opening 5 in the injection molded part 4.

An axially movable sleeve 9 with a conical narrowing 10 provided to forman undercut 11 in the flange 12 of the sprue part 3 and with a furthernarrowing 13 for centered holding of the sprue part 3 is located withinthe displaceable half 2. An axially displaceable ejector 14 is guided inthe sleeve 9 for the ejection of the sprue part 3. The internal diameter18 of the moving half 2 is larger than the external diameter 20 of thebushing 7 and corresponds to the diameter of an axially projecting part15 of the flange 12 of the sprue part 3. As seen in particular in theenlarged view illustrated by FIG. 2, a parting edge 16 is inclined at a45° angle relative to the plane of the opening 5.

Following the completion of the injection molding process, the sleeve 9is moved to the left in the direction of the arrow in FIG. 3. The spruepart 3 is taken along. Due to the adherence of the sprue part 3 to theinjection molded part 4, the flange 12 of the sprue part 3 initiallyundergoes bending stress. The sprue part 3 is held securely by thesleeve 9 as undercut 11 is pressed into the conical narrowing 10. Uponfurther displacement of the sleeve 9 in the direction of the arrow, thesprue part 3 is separated from the injection molded part 4 at theconnecting location 6 along the parting edge 16. According to FIG. 4,the sleeve 9 is further displaced inside the movable half 2 in thedirection of the arrow, into the area of an opening 17. The ejector 14is now displaced in the direction of the arrow to the right and thesprue part is thereby ejected from the narrowings 10 and 13 of thesleeve 9, so that it can drop through the opening 17 from the injectionmolding tool. The narrowing 13 is in contact longer than the narrowing10, so that the sprue part 3 is held securely in the sleeve 9 until theejection process is completed.

The ejection of the sprue part 3 is facilitated by bending of the flange12 resulting from the centered force introduced by the ejector 14, asseen in FIG. 4. Pressure on the undercut 11 of the sprue part 3 isthereby relieved and the ejection of said sprue part facilitated withoutthe generation of harmful dust by the abrasion of the plastic at theundercut.

In this manner the angle of the undercut may be made larger than usual.The ejection of the sprue part is additionally facilitated by thecooling shrinkage.

The ease of removal of the sprue part 3 is enhanced by the contraction19 in the flange 12 at its neck on the sprue part. Due to theconfiguration of the contraction the elasticity of the flange isincreased.

The injection molded part 4, which is seated with opening 5 on thebushing 7, may be removed upon simultaneous or subsequent opening of theinjection molding tool in which the displaceable half 2 is moved to theleft in the direction of the arrow. The injection molded part 4 isthereby released. The removal of the part may be accomplishedautomatically preferably by means of an appropriate device, not shown.The next injection process may be carried out following the closing ofthe tool.

Alternatively, the sprue part and the injection molded part may also beremoved by moving the displaceable half 2 of the tool, the sleeve 9, andthe ejector 14, together with the sprue part 3, and the injection moldedpart 4 to the left in the direction of the arrow. In this case theinjection molded part 4 is held by the sprue part 3 and integrallyjoined therewith.

An automatic discharging device, not shown, now grips and immobilizesthe injection molded part and releases the movement to separate thesprue part from the injection molded part, as described above.

When the sprue part 3 has been separated from the injection molded part4, the discharging device carries out an axial motion in the directionof the stationary half 1. Subsequently, it swivels out around a point ofrotation with the injection molded part 4 from the area of the tool. Asthe movement around the swivelling point takes place outside the toolareas, only a small opening path of the tool is required to remove theinjection molded part 4.

What is claimed is:
 1. An injection molding apparatus comprising:astationary half partially defining a mold cavity; a bushing exhibitingan axial extension beyond said stationary half, said bushing defines asprue funnel and said extension exhibits an external diametercorresponding to and partially defining an internal aperture in saidmold cavity; a displaceable half partially defining said mold cavity andexhibiting an internal diameter defining, circumferentially, a sprueflange cavity; an axially displaceable sleeve within said displaceablehalf partially defining a sprue cavity exhibiting a sprue flangeundercut means for retaining a sprue flange and an axial conicalnarrowing means for retaining a sprue; an axially displaceable means forejecting said sprue guided in said sleeve.
 2. An injection moldingapparatus as in claim 1, wherein said displaceable half internaldiameter is larger than an internal diameter of said stationary half. 3.An injection molding apparatus as in claim 2, wherein said bushing isaxially displaceable.
 4. An injection molding apparatus as in claim 3,wherein said axial extension defines at least 70% of said internalaperture when in a normal position for injecting thermoplastic material.5. An injection molding apparatus as in claim 3, wherein said axialextension and mold cavity are arranged so that upon forming an internalaperture in a molded part a leading edge of substantially 45% is formed.